Why the ARM architecture is shaped the way it is

At this stage Acorn did not exist as a company. Acorn was initially the
trading name of Hauser's company, Cambridge Processor Unit Ltd., Wilson
recalls. "Even before I graduated in 1978 I had an offer of employment,"
Wilson remembers it was for a salary of 1200 pounds a year. "It was a
bad salary even then," reminisces Wilson, but it allowed the pursuit of
an interest in digital design.

"The System 1 we had on Veroboard
and there was a racking system. We had a number of boards; a computer
board, a floppy disk controller board and so on. As Acorn System 1 went
through a number of iterations it built up a following in the industrial
sector, Wilson recalls; a first showing for some of the embedded
capabilities that ARM processors would later have.

It was at
about that time Chris Curry left Sinclair and came to work at Acorn
bringing Steve Furber with him although Furber was not yet an employee,
as he continued studying at the university for a PhD and only worked for
Acorn part time.

Meanwhile Hauser's hopes for Acorn continued to
grow. In 1980 the Acorn Atom home computer was available in either kit
of assembled form, again based on the 6502.

"The Acorn Atom was a
repackaging of the industrial subsystems we had developed," said
Wilson. "It included a BASIC interpreter that I had written, but it also
included some design faults. We needed to do a professional version of
the Atom. Andy Hopper [later Professor Andy Hopper of Olivetti Research
and Cambridge], wanted a workstation to run all the high-end languages
while Chris Curry wanted something just a little better [than Atom] that
would be commercial," recalls Wilson.

"I suggested a two-part design with an I/O processor and a language processor. Proton was the project name," said Wilson.

The
Proton project led to the now infamous pitch by Acorn in 1981 to build a
computer for the U.K.'s national television service, the BBC. The BBC
wanted to commission an affordable home and schools computer on which
they could demonstrate programming and computer science in a series of
broadcasts.

Hauser phoned up Wilson one Sunday in 1981 and asked
if it would be possible to turn the Proton plans into a working
prototype by the following Friday for a visit by the BBC. For once
Wilson told Hauser "no!"

Wilson recalls that Hauser said:
"That's a pity," but seemed to accept the answer. Hauser then phoned up
Furber asking the same question but adding that Wilson had indicated it
might be possible. Furber's initial reaction had been the same as
Wilson's but he agreed that if Wilson thought it doable there was no
harm in trying.

There followed four days of long hours, frantic
work, calling in favors from semiconductor suppliers to get hold of
sample parts, blowing of custom uncommitted logic arrays (ULAs), and
wire-wrapping boards with hundreds of posts and thousands of
connections. "There was a great deal of debugging on the Thursday using
an in-circuit emulator based on an Acorn System 5," recalls Wilson.

This
led to the incident of the machine failing to boot and rejecting all
attempts to diagnose the problem late into Thursday evening until, in
desperation, Hauser suggested disconnecting the emulator. At which point
the prototype sprang into life.

Even on the Friday morning
Wilson was still writing the video software to get the machine to
display a raster but nonetheless the BBC executives who had specified
their home computer should be based on the Z80 processor, gave the
contract to Acorn and their 2-MHz 6502 based design.

For several
years the making of the BBC computer, which achieved a penetration of
80 percent in U.K. schools, was the making of Acorn Computers.

So
what was it about that 8-bitter, that kept Wilson and the Acorn design
team loyal to it. "The key was it was easy to comprehend and to design
stuff around it," said Wilson.

"We'd built up a very good
understanding of the 6502 over the years and we knew it allowed a fast
memory interface," Wilson said.

I give ARM good marks for seeing a development path beyond the Intel line. Intel followed a logical path exploiting their processor line into the home PC market.
ARM saw the long term potential of building a path towards customizable computer processor components to enable smaller runs of targeted processors.
I am impressed with the quality and versatility of the ARM processor line. They may now face more competition from Intel as the PC market begins to phase down. The capability to make building block components has been around for a while, so it will be interesting to see if Intel can come up with a competitive alternative.

Nice article. I was around when much of this was happening and had heard a bit about Acorn computers but was mostly unaware of how it all fit together.
I especially like this statement: 'Wilson concludes: "Hermann Hauser says he gave us the things Intel could never give us, no resources, no time and no money."'
How many great innovations have happened because someone had a job to do and not enough time, resources or money to do it?

From tiny acorns do giant oaks ( ARM ) grow ! Ironic that de-industrialized and uncompetitive England provided a more fertile soil for RISC designs to grow than out here in the desert in the shadow of giant Fabs that are still churning out CISC processors with a billion transistors. But for how long ?

Thank goodness for those 10 Billion transistor giants. Want to try running SolidWorks or AnSYS' Maxwell on even the most powerful ARM8 (if they were even available)? Comparing ARM and Intel architectures is like comparing helicopters with automobiles - they both have areas where they are a great fit - but neither address the full universe of applications.

Shrug---it's more complicated than that, my dear Atlas.
If you squint at published benchmarks like Specmark just right, the more complex ARM models gets comparable or better performance per GHz. The real reason why x86 architecture runs your favourite applications is, as you pointed out, because they are not available for ARM, because Wintel.
This is changing slowly: there are reports that PC sales are crashing, and the Wintel shiny front wall starts showing cracks. Will SolidWorks be available and usable on Android any time soon? Probably not, but the reason is not 'more transistors' or better architecture on x86.

De-industrialised? Uncompetaive? Take a look at this PWC report:
http://www.pwc.co.uk/assets/pdf/ukmanufacturing-300309.pdf
"There is a widespread assumption that the final demise of manufacturing in the UK is only
a matter of time. But this is simply not so. The facts tell a different story:
• Output of British manufacturing reached an all-time high in 2007, even adjusted
for inflation
• The UK is the world’s 6th largest manufacturer with strong positions in certain key
industries, e.g. a 15% global market share in Aerospace
• UK Manufacturing achieved a 50% increase in labour productivity from 1997-2007"

@Jack...go easy on our friend chipmonk. For the past 50 years public education here in the States has atrophied the curricula so badly that many "students" couldn't find England on a map. I work with younger BS/MSEEs who cannot even compose a coherent paragraph...they're not stupid, in fact many are quite bright, it's just that the system has failed them.

They're all good. In today's world of SoC the architecture of the CPU usually, but not always, takes second or third place when comparing features like the peripheral set, Pd and package alternatives. Afterall, all Boole, et al left us with is AND and NOT...everything else is but a variation on a theme.

Was the ferranti ULA a sea-of-gates with a metal top layer - or was it actually an eraly FPGA ?
Google found this... chances are google run on thousands of Intel processors.... ironic.
http://tinyurl.com/ferranti-ULA-problems-delay

ARM in early 90s was not so much in advance of other chips, especially MIPS, and MIPS sold a lot more. Hauser forgot one other essential thing he gave ARM: no customers. So when the mobile industry was ready to build SOCs, there was ARM with a suitable chip, fabless portable design, and no other business model to distract them. It was their one lifeline, and it proved to be a bonanza. The dominant competitor capable of providing a choice at the time, MIPS, was very successful elsewhere and by the time phones were big enough business for them to become concerned, ARM had a lock.
These days, ARM is part of SOCs with billions of transistors just like x86. The difference is in balance: in the ARM ecosystem the die is shared with IP like GPUs, modems, and half a dozen other blocks which have formed a healthy ecosystem. x86 is elbowing in: will it have more success now than MIPS 15 years ago?

The core of ARM was that it had to be cheaper than an off the shelf processor or else Acorn went bust. Thats why when Robin Saxby took over he needed to find a customer for the core as Acorn were not buying enough to keep going. Nokia told people to licence this as they were not paying for someone elses IP as it would mean GSM handset were going to be too expensive. TI licenced it and there history was written

What an odd reply!
I think the point is that this is story about people, and at some point in the timeline the main character in the story had a sex-change, which is a remarkable thing, making it a glaring omission from the story.

The first project I worked on used the BBC microcomputer to do 3D spherical trigonometry using radio signals in real time to compute location - sort of a poor man's GPS.
What was astonishing was how fast floating point code written in Basic would run on this 8 bit 6502. I guess this was due to hand-crafted C or assembler floating point routines embedded in ROM but I still don't really understand how they made interpreted code run so quickly.

Yes I recall my days at ACORN and interaction with Herman, Roger and Steve.
I even recall Herman & my discussion about the daily calendar product that was to have 16 charter LCD displays from Hitachi and use there RT chip who we were connected with very closely for memory parts for our BBC and Electron
What a group and what an innovation company from the day one joined…looking back it was love of my life as I have not come across another company like it for pushing the envelope on daily bases. Gopro may today’s such leader
At ACORN, each one of us was doing 5 projects at any one time from network to making mouse using CCD sensors. I even tacked a research storage device where there head moved spirally to the center with removable 2in cartridge. This is before the 3.5in Discs became the norm
Oh I still have an Electron and it still boots-up and runs Basic, 6502 Assemble like no other product today to hack code in 5min and run 2MHz assembly in Basic. What a concept
Only good memories of my carrier and working with Herman, Roger Wilson (Sophie Wilson) and off course Chris Curry who we started a company called GSI out of his 40 bedroom Manson in Croxton Village together with Ram Banerji
ZahidOh I still have an Electron and it still boots-up and runs Basic+6502 Assemble like no other product today
znajam@msn.com

Back in the days I worked at an early computer store, we ran a set of BASIC benchmarks originally published in BYTE magazine on the other computers in our store after we found our numbers agreed with BYTE's list of computers they ran on.
I further wrote a program to sort and rank the results (this being the days before VisiCalc) and one item I tracked (but did not sort on) was the processor the syetem used.
Were were looking to see who wrote the better BASIC interpreter, but were surprised when we saw processor stratification instead.
The 1 MHz 6502s were faster than the 4 MHz Z-80s, followed by the 4 MHz 8080s.
Looking into it, aside from the pipelined instructions, loads took 3 cycles on a 6502 while the Z-80 took 6, and the condition code was set automatically by the load instead of requiring another instruction like the Intels, I felt it was the reduced number of registers (6502 A, X & Y) (Z-80 A, BC, DE, HL + mirror set) helped as well. From my experience programming S/370 Assembler, I had seen too many cases of registers being swapped around, just to make use of register sets already being used, or for special purpose use, which in the end did not do much but chew up clock cycles. Given the reduced number of registers on the 6502, you made use of them for good purpose, you didn't calculate a value and let it hang in a register for K's of execution later until you whipped it out of seemingly nowhere (making debugging harder as well).
That made the 6502 far more efficient than the Intel processors, even though they ran at 4x the clock rate.

Nobody gives rat's ass what you used registers for on any CPU. ARMH is successful because customers can design targeted SOCs in 1/4 the time it take Intel to provide a reference design for what they define as the next mobile CPU one year too late.

Extremely well done story, Peter. I enjoyed it. "It took four clock ticks to run a (68000) memory cycle." That told me something I didn't know about the evolution and inherent advantage of the ARM architecture, which I assume took one tick. Charlie Babcock, InformationWeek

Most processors at the time of the 68000 were CISC or multiple clocks per instruction. It wasn't until later when power and size for embedded applications were the drivers that brought RISC (ARM) type processor to the forefront.

About the writing...There appears to be a paragraph or two missing at the beginning of the article that sets the scene. The first sentence starts with "Most computers at that time". I guessed the setting was the late 1980s.
I was trying to understand who the players were. Mentioned were " Wilson, Furber and the rest of the team" but no context.
I noticed an inaccuracy about the IBM PC processor. The first PC used the 8088, the 8-bit data-bus version of the 8086.
Wait...I just noticed at the bottom of the article a page button where it says page 3 of 3. I arrived from a LinkedIn discussion group. I don't see paging buttons at the top of the article.

This is more a *very* brief history of the people responsible for the design of the ARM architecture than it is a history of the conception of the design itself. I walk away from reading this article wanting to know way more about the design philosophy and design choices that were made regarding how the architecture came together. To me, the instruction set and the programmer's model, and the thought process going into their design, constitute much of what I would consider the "shaping" of the architecture. That merited one short paragraph.
That the ARM architecture had to be simple, compact, fast, and have low power consumption is a little obvious. That the framers of the ARM architecture have their roots in the 6502 is little more than interesting trivia.